The system and techniques described herein relate generally to radio frequency (RF) communications and, more particularly, to antennas and antenna systems for RF communications in hear-field sensing applications such as, but not limited to, radio frequency identification (RFID) systems.
As is known in the art, radio frequency identification systems are typically wireless, non-contact systems that utilize radio frequency electromagnetic fields to transfer information from an RFID card or tag to a reader for the purposes of automatic identification and/or tracking. RFID systems are used in a wide variety of different applications including, but not limited to, evacuation management, security systems, asset tracking, manufacturing, and people (e.g., students, employees) tracking.
As is also known, electrically small loop antennas, that is, those having electrical dimensions less than about one-eighth of a wavelength, often used in near-field sensing applications, have limited bandwidths, commonly less than a one or two percent of the operating frequency. Many applications require that the loop antenna operate over a bandwidth which is wider than the naturally occurring operating bandwidth of loop antennas. To expand the operational bandwidth of such loop antennas, it is common to reduce the antenna efficiency. Thus, in some applications, a trade-off must be made between operating bandwidth and efficiency of the loop antenna. One such application is in the reading of RFID cards that conform to the applicable International Standards Organization standard, ISO-14443.
The antenna used as part of an interrogation system for typical RFID cards must have a bandwidth sufficient to provide reasonable gain at the card's response sub-carrier frequency, which is 483 kHz away from the carrier signal that provides the card's power. This generally limits the maximum antenna Q to no more than about 40, which limits the distance at which a card can be read for a given interrogation power.
Other applications abound where small antennas are desirable but have limited utility because of their limited bandwidths.
What is needed is a high efficiency, wide bandwidth yet physically small antenna system for use in RFID and near-field sensing applications that enable the transmission of short duration pulses and/or the rapid and efficient transfer of high-bandwidth data.